This invention is in the area of biodegradable thermoplastic materials.
A number of nonbiodegradable plastics such as polyethylene, polypropylene, polystyrene, and polyurethane are now used extensively in place of metal and paper products for many applications, especially those where cost, durability, ease of manufacturing, availability of material and convenience are major considerations. One of the biggest problems with these plastics, however, is with disposal, since they have very low rates of degradation, if any. The problems with persistence of styrofoam hamburger containers and plastic six-pack holders are well known.
A number of biodegradable polymers which degrade by enzymatic or hydrolytic action have been proposed as substitutes for nonbiodegradable plastics, including poly(glycolic acid), poly(lactic acid) and copolymers thereof, polycaprolactone, poly(hydroxybutyrate), starch and cellulose. These materials have been suggested for a diverse range of applications, from biodegradable garbage bags to implantable drug delivery devices. Cost is a major consideration in many non-medical applications, however, since many of the synthetic biodegradable polymers are expensive to make, and are difficult to process easily or with the desired mechanical or physical and chemical properties.
Poly(.beta.-hydroxybutyrate) is an example of a degradable, biocompatible, thermoplastic polyester made by microorganisms. Engelberg and Kohn, Biomaterials 12, 292-304 (1991), evaluated the thermal properties of poly(.beta.-hydroxybutyrate) and found that the melting temperature, at 171.degree. C., is very close to the upper limit of its thermal stability, making the material difficult to melt process into useful articles. Poly(.epsilon.-caprolactone) is an aliphatic polyester that degrades by a hydrolytic mechanism under physiological conditions. However, poly(.epsilon.-caprolactone) has a glass transition temperature of about -60.degree. C. and is therefore always in a rubbery state at room temperature.
Starch has been evaluated for use as a polymer for use in manufacturing articles. It is very susceptible to enzymatic digestion by the enzyme .alpha.-amylase which attacks the .alpha.-D (1-4) glucosidic linkages. These linkages can be cleaved in either physiological or environmental conditions. However, starches form weak and brittle products. Starch is not thermoplastic by itself and therefore cannot be melt processed into useful products by extrusion, compression molding, injection molding, calendaring, or fiber spinning without the addition of significant quantities of plasticizers.
Probably the greatest disadvantage of using starch as a manufacturing material is that due to its hydrophilic nature, the water content of an object made from starch is difficult to control. This can lead to undesirable changes in the physical properties of the object, since physical properties are strongly tied to water content.
Blends of non-biodegradable plastics and cheap biodegradable fillers, such as starch, have been investigated. ECOSTAR.TM. is a blend of a starch that has been modified to render it hydrophobic and a traditional thermoplastic polymer such as linear low density polyethylene, high density polyethylene, polypropylene, or polystyrene (Food Engineering p. 29 (July 1988); Maddever and Chapman, "Modified Starch Based Biodegradable Plastics", ANTEC '89, Soc. Plastics Eng. Conference Proceedings pp. 1351-1355 (May 1-4, 1989)). An autooxidant is added to the blend to accelerate biodegradation. While these thermoplastics offer some advantages over other materials, they still incorporate traditional polymers that degrade very slowly or not at all.
Holland, Yasin, and Tighe, Biomaterials 11, 206-215 (1990), blended starch with hydroxybutyrate-hydroxyvalerate copolymers to manipulate the hydrolytic degradation process of the copolymer. Polysaccharides were added to the copolymer compositions to accelerate porosity development and thus enhance degradation rate.
A. T. Gros and R. O. Feuge, J. Amer. Oil Chemists' Society 39, 19 (1962), have derivatized amylose with fatty acids to modify the thermal and mechanical properties of starch. Similarly, I. A. Wolff, D. W. Olds, and G. E. Hilbert, Ind. Eng. Chem. 43:4, 911 (1951), have esterified corn starch, amylose, and amylopectin with various fatty acids to produce starch derivatives with varying thermal and mechanical properties.
Ferruti, Tanzi, and Vaccaroni, Makromol. Chem. 180, 375-382 (1979), prepared imidazolides derived from the succinic esters of starch and dextran, which undergo exchange reactions with alcohols or amines to give the corresponding polymeric esters or amides. However, these succinic esters were not designed to be thermoplastic (e.g., processable as a melt) or to have enhanced mechanical integrity.
There remains a need for a thermoplastic material that is easily processable into articles of manufacture that have sufficient physico-mechanical properties for the intended use, yet which are biodegradable in a reasonable timeframe into relatively nontoxic materials.
It is therefore an object of the present invention to provide a biodegradable thermoplastic material that is capable of being melt processed into articles that degrade under normal environmental conditions into relatively nontoxic materials.
It is a further object of the present invention to provide a method for making biodegradable materials which can be readily processed into articles of manufacture which exhibit a variety of mechanical and physical properties.